In what form is CO2 carried primarily in blood?

Carbon dioxide (CO2) is primarily carried in the blood in the form of bicarbonate (HCO3-). This occurs through a biochemical process that begins when CO2 diffuses from body tissues into red blood cells, where it reacts with water to form carbonic acid (H2CO3), catalyzed by the enzyme carbonic anhydrase. Carbonic acid then dissociates into bicarbonate and hydrogen ions. The bicarbonate ion can then move out of the red blood cells into the plasma, where it accounts for about 70% of the total CO2 transported in the blood.

Carrying CO2 as bicarbonate is crucial for maintaining the acid-base balance in the body and allows for efficient transport of this waste gas from tissues to the lungs, where it can be exhaled. The other forms, such as being dissolved in plasma or bound to hemoglobin, account for much smaller percentages of CO2 transport.